The invention relates in general to surface-wave resonator filters and, in particular, to a surface-wave resonator filter on a piezoelectric substrate with a reflector structure having metallization strips. At least one interdigital transducer is provided on the substrate and wave propagation occurs in a direction perpendicular to the metallization strips.
U.S. Pat. No. 4,623,855 assigned to the present assignee is incorporated herein by reference. Also incorporated herein by reference is the prior art discussed in the background of U.S. Pat. No. 4,623,855, specifically, the article entitled "IEEE Transactions on Sonics and Ultrasonics", Volume SU-25, No. 3 (1978), pp. 138-146 and U.S. Pat. No. 4,340,834.
From "IEEE Transactions on Sonics and Ultrasonics", Volume SU-25, No. 3 (1978), pages 138-146, a surface acoustic wave resonator filter is known whose reflector structure, as shown in FIG. 1 therein, has, instead of continuous metallization strips, a field of metallization points in a regular waffle-iron distribution. The metallization spots succeeding thereafter in the direction transversely to the wave propagation direction replace a respective metallization strip of conventional reflector structure. The individual metallization spots have dimension on the order of magnitude of the wave length or fractions of the wave lengths of that particular wave for which the respective reflector structure is intended. Given a strip length of e.g. 100 wave lengths, then for each individual strip, 200 interruptions result. FIG. 4 of this publication shows the result; namely, that secondary lobes of the frequency curve of a filter designed in this manner are still only poorly pronounced. This is attributed to the fact that in the direction of the individual metallization strips, no electric current flow can develop which leads to transverse modes based thereon and has as a consequence signifcant changes in the transit or propagation time speed of the desired propagating longitudinal wave.
An extremely frequent interruption of the customarily employed metallization strips, demanded in accordance with the above-cited publication, signifies an additional high technological design expense for the manufacture of such a filter.
It has already been investigated (U.S. Pat. No. 4,340,834) to what extent the result is made poorer if only four interruptions each given a number of fingers of the reflector structure are provided. These interruptions, in the case of all fingers provided for this purpose, have been localized so as to mutually correspond; i.e. all interruptions lie adjacent in one row. In this regard, see FIG. 1 and 4a of the U.S. Pat. This principle of corresponding subdivisions which are precisely adjacent one another, is also present in the case of all further examples of this U.S. Pat.
In contrast with arrangements of the above-discussed publications "IEEE Transactions . . . ", in the case of the arrangements of the U.S. Pat. No. 4,340,834, the occurrence of transverse modes has again been consciously taken into account. Due to the interruptions, on account of the regularity of their arrangement directed parallel to the wave propagation of the longitudinal wave, adjacent strips with propagation speeds different from one another develop therein (see U.S. Pat. No. 4,340,834, column 5). With the occurrence of these strips, the harmful effects of the occurring transverse modes are decreased therein. However, an additional attenuation for the desired wave occurs.
According to FIG. 4B of the U.S. Pat. No. 4,340,834 the result attained therein is such that, in the curve of the attenuation progression, no side lobes of the transverse modes can be recognized any longer. On account of the logarithmic scale of the attenuation curve, however, only very considerable side lobes would be recognizable therein. However, the progression of the envelope or group delay is decisive, which, as a linear function, permits the relevance of still present side lobes to be significantly more clearly recognized, and, in the last analysis, it is this envelope delay which is of decisive significance for the signal transmission with a filter of this type.
A reflector structure designed for another purpose, namely for finger weighting, is shown in FIG. 1 in "Electronic Letters", Volume 16 (9/10/1980), pp. 793-794, incorporated herein by reference. It is provided therein, for a portion of the reflector structure, instead of providing reflector elements which are continuous over the entire width of the structure, to omit the weighting corresponding to the length components of this element provided for the respective reflector element. These omissions are distributed over the entire length of the respective reflector element so that a single weighted reflector element viewed in the direction of the individual reflector element, i.e. transversely to the wave propagation of the surface acoustic wave, consists of the reflector element pieces between which (in their transverse direction) considerable distances exist. The reflector element pieces all have an equally great length and there above-mentioned distances (in a transverse direction) from one another, correspond to the prescribed weighting of the individual reflector element, are more or less large. In the case of this known reflector structure, the individual strip-shaped reflector elements and also the reflector element pieces are grooves etched into the surface of the substrate, i.e. no metallization of the surface is present in this regard. Therefore, electric currents occurring as in the case of an arrangement according to the initially described publications transversely to the wave propagation direction (i.e. such currents which lead to transverse modes) are impossible in this embodiment with grooves. Thus, the weighted reflector elements consisting in the case of this arrangement of groove-shaped reflector element pieces exhibit several interruptions but have equal-length reflector element pieces, respectively.